Chronic daily headache: old problems, new vistas

In 1988 the problems concerning chronic daily headache (CDH) were neglected by the classification of the International Headache Society (IHS). More than ten years later, this issue is still debated, also in light of the foreseen revised classification. Several terms have been used to define the clinical picture of CDH, and different criteria have been proposed for the diagnosis of these forms. In most cases, CDH appears to evolve from an episodic migraine, but the temporal limits between an episodic and a no-longer episodic form of migraine are questionable. A decreased threshold for headache recurrence in CDH is currently hypothesized, and it may be due to either an impaired control system or a sensitization of the trigeminal neurons, occurring regardless of the original nature of headache. The identification of genetic alterations and neurobiological changes underlying the different forms of CDH may greatly facilitate any nosographic and therapeutical approach to this broad spectrum of disorders.